This invention deals generally with plasma film removal on semiconductor devices and, more specifically, with an improved design for a cathode utilizing multiple contact areas that is external to the reaction chamber.
Plasma processing techniques for patterning or depositing thin films on work pieces, such as semiconductor wafers, have been utilized for approximately 20 years. Plasma was first utilized in semiconductor processing for photoresist stripping. The development of very large scale integrated (VLSI) devices has been a more recent catalyst that has prompted further refinements in plasma techniques and processes.
Batch systems, in which large numbers of wafers are dry etched or stripped in an apparatus at one time, have been used commercially to process semiconductor devices for a number of years. Typical of these systems is the barrel type of reactor or etcher that is so named because of the reactor's shape. Barrel systems are volume-loaded and can accommodate many wafers. They have been used primarily for photoresist stripping. However, they generally do not provide a high degree of etching precision or etch uniformity.
An alternative to batch systems is the use of single wafer etchers or strippers. Initial single wafer etchers operated at low pressures and suffered from a relatively slow etch rate, which is the removal rate of a material at a specified location on the wafer surface. As the semiconductor industry matured, however, a trend developed toward using larger size wafers with greater pattern density. These larger wafers are more valuable and have become increasingly more desirable. However, larger size wafers require closer process control during etching or stripping operations.
Single wafer etchers or strippers also have been found to be better suited than batch systems for automation, such as the automatic loading or robotic loading of the individual wafers. Compared to batch systems, single wafer etchers or strippers can be more compact, occupying less space. However, most previous single wafer systems suffered from the aforementioned slow etch rate and from microcontamination problems.
Contamination frequently occurred in the prior low pressure, high power density plasma strippers primarily because of the use of internal metal electrodes. This type of a system developed plasmas, such as oxygen, which generated high D.C. bias levels on the cathode and produced high oxygen ion energy levels. Upon collision with the metal electrodes, typically formed of aluminum, the high energy level oxygen ions caused the electrodes to sputter, with some of the sputtered electrode material landing on the wafer. Where aluminum was used as the material of the electrode, an aluminum oxide film or oxidized aluminum particles were present as contaminants.
Elevated wafer temperatures due to processing in prior single wafer stripping systems also presented the problem of mobile ion and impurity migration. In this instance mobile ions, for example sodium ions, in the photoresist migrate from the photoresist through the silicon dioxide films to the silicon dioxide - silicon interface or through the patterned film to the film's interface with the underlying structure. This latter phenomenon produces unacceptable electrical characteristics when such wafers are used in integrated circuits.
These problems are solved or substantially reduced in the design of the single wafer photoresist plasma stripper of the present invention.